Fiber-containing materials, such as paper, paperboard or the like, are usually formed in continuous processes, producing continuous webs in the form of sheets or rolls. Although such webs are formed from a slurry of randomly oriented fibers, the machines used for formation of the webs tend to align the fibers in the machine direction (MD), that is, in the direction in which the webs are formed. The alignment of fibers in the machine direction imparts different properties to the web in the MD than in the perpendicular or cross-machine direction (CD). For example, the web may tear more easily, but more evenly, in the MD than in the CD.
For many applications, it is desirable to keep random fiber orientation anisotropy (FOA) to a minimum, and there is a need to monitor samples of the web to measure the degree of fiber orientation anisotropy. Some applications, on the other hand, may make advantageous use of a certain degree of anisotropy, and in such cases also, it is necessary to monitor samples of the web to ensure that the degree of fiber orientation anisotropy falls within a specified range. Further, orientation ratios across the web can be important to provide product uniformity.
Presently, fiber orientation anisotropy measurements are conducted by direct observation, x-ray diffraction, and laser transmittance techniques. The first determinations or fiber orientation distributions were made on sheets with a small added portion of dyed fibers. The angles of orientation of thousands of dyed fiber segments were manually measured. This provides a direct measure of the orientation, but it is tedious, requires specially prepared sheets, and only measures surface properties. Anisotropy in x-ray diffraction patterns is also used as a laboratory indicator of FOA. Recently, the diffraction pattern of transmitted laser light has been used to infer FOA on very thin sheets. On sheets of moderate thickness, FOA is estimated from the pattern of forward scattered light. Laboratory and on-line instruments based on the scattering principle have been commercially available. The microwave technology, described here, is complementary to the other techniques. A major advantage is that microwaves penetrate very heavy sheets. Also, microwaves have been heretofore used extensively to measure paper moisture content on-line, and a moderate modification of this established technique could lead to fiber orientation as well as moisture content information. However, there exists a need for rapid, simple tests for detecting FOA that are nevertheless reliable and repeatable, and it is a primary object of the invention to provide methods and apparatus for measuring FOA.